As the density and complexity of microcircuits continue to increase, the photolithographic processes used to print circuit patterns becomes more and more challenging. Previous technologies and thinking in the art has required denser and more complex patterns to achieve the formation of the denser circuits consisting of smaller pattern elements packed more closely together. Such patterns push the resolution limits of available lithography tools and processes and place serious burdens on the design and quality of the photomasks used therein. To push the resolution limits, advanced photomasks are designed using various Resolution Enhancement Techniques (RET). Optical Proximity Correction (OPC) is one such technique. With OPC the photomask patterns are modified in various ways to help ensure that the printed pattern has good agreement to the original desired pattern. These photomask pattern modifications can include perturbations to the size of main pattern features, the addition of serifs to pattern corners, and the addition of Sub-Resolution Assist Features (SRAFs).
But even with such crafty approaches optical lithography is believed to have certain inherent resolution limitations which define the maximum utility of the lithographic patterning approaches. One of these limitations is defined by the well-known Rayleigh criteria. Techniques such as OPC and SRAF promise advancements in optical lithography ever closer to the elusive Rayleigh resolution limit of k1=0.25. Improving lithography resolution by using RETs to approach the ultimate physical resolving power of a given lithography toolset is often cheaper and timelier than installing a higher-resolution toolset, but it still is costly. In addition to increased reticle cost and rising process complexity, significant resources must be dedicated to the development and implementation of technology-computer-aided design (TCAD) solutions to manage the escalating complexity of RET-related chip layout modifications. But for all of these improvements there is a finite limit to the resolution possible with optical lithography.
In a simplified approximation of coherent illumination, the resolution R of a lithography system is conventionally quoted in terms of the smallest half-pitch of a grating that is resolvable as a function of wavelength (λ) and numerical aperture (NA), as expressed by Rayleigh's equation,
  R  =            k      1        ⁢          λ      NA      
As is known, the resolution R of an optical system defines its capability for distinguishing closely spaced objects. Accordingly, resolution defines the minimum line width or space that a lithography system can print.
As is also known, standard lithography techniques involve forming a layer of a photoresist material on a substrate and then exposing it to an optical beam patterned by a photomask in a single exposure to obtain high throughput patterning of the photoresist layer. The photoresist layer is then developed leaving a patterned surface on the substrate which is used create patterned layers on the substrate in accordance with any of a number of well-known fabrication procedures.
Heretofore, resolution improvements were obtained by decreasing the exposure wavelength (λ), increasing the numeric aperture (NA), or decreasing k1. Exposure tool wavelengths (λ) have been progressively shortened from g-line and i-line sources, to 248 nm KrF lasers, 193 nm ArF lasers, and even 157 nm lasers to fabricate at steadily smaller CD nodes. However, each shortening of wavelength is becoming harder to achieve. The same can be said for increasing system NA with so-called immersion systems defining the highest NA systems yet devised. Phase shift and off-axis illumination technologies can be used to provide reductions in k1. However, all of these improvements cost money and increase process complexity.
Additionally, the prior art technologies are all faced with a certain finite limit on resolution which eventually will become too expensive to overcome or impossible to overcome. Thus, although the prior art techniques are generally suitable for the purposes for which they are intended, they suffer from certain limitations, not the least of which being a finite limitation on the pattern density at which such patterns can be printed using photolithographic processes.
Accordingly, the embodiments of invention present substantial advances over the existing methodologies and overcome many limitations of the existing pattern fabrication arts. These and other inventive aspects of the invention will be discussed herein below.